In an optical storage system, after a light spot emitted by an optical pickup head is irradiated on lands and pits of the tracks of an optical disc, a light beam reflected from the optical disc is received by a photo detector IC (PDIC). According to the reflected light beam, the photo detector IC generates a radio frequency signal (RF). After the radio frequency signal is processed by a read channel of the optical storage system, a sliced radio frequency signal (SLRF) indicative of the data of the optical disc and a channel clock (PLCK) are acquired.
FIG. 1A is a schematic functional block diagram illustrating a read channel of an optical storage system according to the prior art. As shown in FIG. 1A, the read channel comprises an analog-to-digital converter (ADC) 101, a timing recovery unit 110 and a signal slicer 120. The timing recovery unit 110 comprises a high-pass filter (HPF) 103, a digital equalizer (EQ) 106, a zero-crossing reference level generator 108, and a digital phase locked loop circuit (DPLL) 109.
Since the read channel of the optical storage system is implemented by a digital signal processing circuit, the radio frequency signal RF outputted from the optical pickup head should be sampled by the analog-to-digital converter 101. That is, the radio frequency signal RF should be converted into a digital radio frequency signal RF_d. After the digital radio frequency signal RF_d is inputted into the timing recovery unit 110, the DC component of the digital radio frequency signal RF_d is filtered off by the high-pass filter 103, then the gains of various frequency values are adjusted by the digital equalizer 106. A gain adjusted signal generated by the digital equalizer is inputted to the ZCRL generator 108 to generates a zero-crossing reference level ZCRL. The digital phase locked loop circuit DPLL 109 generates a re-timing output signal RTO and a channel clock PLCK in response to the gain adjusted signal and the zero-crossing reference level ZCRL.
The re-timing output signal RTO is further received by the signal slicer 120. Moreover, according to a slice level (SL), the signal slicer 120 generates a sliced radio frequency signal SLRF indicative of the data of the optical disc. The signal slicer 120 is usually referred as a Viterbi decoder or a partial response maximum likelihood circuit (PRML circuit).
FIG. 1B is a schematic timing waveform diagram illustrating the channel clock PLCK and the sliced radio frequency signal SLRF processed in the read channel of the optical storage system. As shown in FIG. 1B, in a case that the pulse period of the channel clock PLCK is T, the sliced radio frequency signal SLRF includes a plurality of successive pulses with the lengths 3T, 5T, 10T, 3T. After the sliced radio frequency signal SLRF is decoded by a subsequent decoding circuit (not shown), the data of the optical data are realized.
Take a DVD disc for example. The track of the DVD disc has land and pit lengths of 3T to 14T. During the process of burning or imprinting the lands and pits, if some problems occur, the sliced radio frequency signal SLRF may be erroneously written. The above erroneous situations usually occur at the short-T parts. The short-T parts indicate the pulses with the lengths 3T, 4T or 5T. Whereas, the pulses with the length longer than 5T are denoted as the long-T parts. Generally, the pulse with the length 3T has the most possibility to incur the erroneous situations.
FIG. 2A schematically illustrates associated signals of an optical disc with a pit-land symmetry configuration. As shown in FIG. 2A, alternate pits 102 and lands 104 of the track have lengths of 5T, 3T, 3T, 4T and 3T, respectively. For an ideal optical disc, for example a ROM disc to be imprinted by a mold or a recordable disc to be burned by laser, the land and the pit with the same length are symmetric. For example, as shown in FIG. 2A, the lengths of the 3T land and the 3T pit are identical. The optical disc with the land-pit symmetry configuration is also referred as a symmetry disc or an ideal etching disc.
In a case that the track of the optical disc has the land-pit symmetry configuration, the radio frequency signal RF is vertically symmetrical with respect to the slice level SL. Moreover, according to the slice level SL, the sliced radio frequency signal SLRF indicative of the data of the optical disc is generated.
FIG. 2B schematically illustrates associated signals of an optical disc with a negative asymmetrical configuration. Similarly, alternate pits 102 and lands 104 of the track have lengths of 5T, 3T, 3T, 4T and 3T, respectively. If the mold is frequently used in this type of optical disc, the length of the pit becomes longer than the length of the land. Otherwise, if the laser power for burning the optical disc is too high, the length of the pit becomes longer than the length of the land. For example, as shown in FIG. 2B, the length of the 3T pit is longer than the length of the 3T land. This optical disc is also referred as a negative symmetry disc or an over-etching disc.
In a case that the track of the optical disc has the negative asymmetrical configuration, a negative DC offset is induced in the radio frequency signal RF causing the slice level SL not properly slicing the radio frequency signal RF. Under this circumstance, the short-T part (e.g. the 3T or 4T part) is possibly lower than the slice level SL, and thus the sliced radio frequency signal SLRF may be erroneous. For example, as shown in FIG. 2B, the 3T part of the sliced radio frequency signal SLRF fails to be actually sliced.
FIG. 2C schematically illustrates associated signals of another optical disc with a positive asymmetrical configuration. Similarly, alternate pits 102 and lands 104 of the track have lengths of 5T, 3T, 3T, 4T and 3T, respectively. If the mold is frequently used in this type of optical disc, the length of the pit becomes shorter than the length of the land. For example, as shown in FIG. 2C, the length of the 3T pit is shorter than the length of the 3T land. This optical disc is also referred as a positive asymmetry disc or an under-etching disc.
In a case that the track of the optical disc has the positive asymmetrical configuration, a positive DC offset is induced in the radio frequency signal RF causing the slice level SL not properly slicing the radio frequency signal RF. Under this circumstance, the short-T part (e.g. the 3T or 4T part) is possibly higher than the slice level SL, and thus the sliced radio frequency signal SLRF may be erroneous. For example, as shown in FIG. 2C, the 3T part of the sliced radio frequency signal SLRF fails to be actually sliced.
From the above discussions, the defects in the pits 102 and the lands 104 of the track may result in asymmetry of the sliced radio frequency signal SLRF, and thus the possibility of causing the erroneous sliced radio frequency signal SLRF increases.
For achieving a better symmetric property of the radio frequency signal RF, the digital equalizer 106 of the read channel is employed to adjust the gain of the short-T part. FIG. 3 schematically illustrates an approach of using the digital equalizer to adjust the symmetric property of the radio frequency signal RF according to the prior art. Take a DVD disc at 1× speed for example. The frequency of the 3T-14T parts is in the range between 4.36 MHz and 0.93 MHz. In particular, the frequency of the 3T part is 4.36 MHz, and the frequency of the 14T part is 0.93 MHz.
Normally, the gain function of the digital equalizer 106 is expressed by the curve I. As the gain of the curve I at the 4.36 MHz is increased, the amplitude of the 3T part is suitably amplified to enhance the symmetry of the radio frequency signal RF. Moreover, if the 3T part of the radio frequency signal RF has inferior symmetry, the gain function is adjusted to the curve II. Obviously, although the gain increase AG at the 4.36 MHz is effective to amplify the amplitude of the 3T part. However, according to the curve II, the gains of 4T˜14T parts are also increased causing the radio frequency signal RF suffering from distortion.
FIG. 4 schematically illustrates an approach of using an asymmetrical compensator to adjust the symmetric property of the radio frequency signal RF according to the prior art. In this approach, an asymmetrical compensator (not shown) is arranged between the analog-to-digital converter 101 and the high-pass filter 103 to receive an asymmetrical radio frequency signal RF and convert the asymmetrical radio frequency signal RF into a symmetrical radio frequency signal RF. In a case that an asymmetrical radio frequency signal having an asymmetrical eye diagram 210 is inputted, by using a compensation curve 200 of the asymmetrical compensator, the asymmetrical compensator may output a symmetrical radio frequency signal having a symmetrical eye diagram 220. In other words, the use of the asymmetrical compensator can compensate the shift of the short-T part. However, the long-T part fails to be effectively compensated by the asymmetrical compensator and the back-end digital circuit needs to be re-designed.